WIP: async transaction execution

include/caosdb/logging.h

include/caosdb/protobuf_helper.h

include/caosdb/transaction.h

include/caosdb/transaction_status.h

src/caosdb/entity.cpp

src/caosdb/protobuf_helper.cpp

src/caosdb/transaction.cpp

src/caosdb/unary_rpc_handler.cpp

test/test_issues.cpp

test/test_transaction.cpp